Methods and devices for haptic communication

ABSTRACT

A haptic stimulator includes a multilayer sheet with a piezoelectric or electroactive polymer layer adapted to mechanically deform upon application of voltage, the multilayer sheet secured to a substrate, and a source of electrical stimulation coupled to drive electrodes on the polymer layer with an AC signal to vibrate the polymer layer. In particular embodiments, the polymer contains polyvinylidene fluoride, and electrodes are patterned to control local electric fields. Another haptic stimulator has first and second electrodes with an air gap and an insulating sheet between first and second electrodes, with an AC voltage driver connecting to the electrodes. In a method of providing haptic stimulation to skin an alternating current supply drives first and second electrodes, the electrodes disposed upon either a piezoelectric or electroactive polymer sheet, vibrating the polymer layer by driving the electrodes; and coupling vibrations of the polymer layer to the sensate skin.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 15/880,907 filed Jan. 26, 2018, which claimspriority to U.S. Provisional Patent Application No. 62/450,993 filed 26Jan. 2017, the entire contents of which are incorporated herein byreference.

BACKGROUND

Normal skin has sensors adapted to sense heavy and light pressure,vibration, heat, cold, and pain; collectively we call these sensations,and the ability to sense movement of zones of pressure, the sense oftouch. Skin having normal sense of touch is referred to herein assensate skin. Touch is important to normal life; in particular it allowspeople to grasp objects with pressure sufficient to avoid dropping theobjects yet avoid crushing them. It also allows movement in the dark,alerts people to unwelcome assault by insects or infections, and allowsproper positioning of fingers on keyboards without need to look at bothkeyboard and fingers.

Upper limb amputees wearing prosthetics often miss the sense oftouch—this is why amputees given the rarely-used Kreukenbergforearm-stump-splitting procedure often reject prosthetics except forformal events where visual appearance is critical. They often find thatnot only is grip strength of the modified-forearm pincers strongcompared to typical prosthetics, but the ability to feel with sensateskin of bare residual forearms is helpful to them.

Haptic communications is communication through these sensors of skinthat provide the sense of touch.

Amputees may benefit from adding a sense of touch, or hapticcommunications, to prosthetic devices.

Most modern entertainment stimulates visual and auditory senses. Only afew theaters, such as “4-D” theaters at Universal Studios amusementparks in Orlando, Fla., are equipped to stimulate senses other thanvision and hearing through air and scent puffers, seat-moving actuators,and water sprinklers—the sense of touch is largely unused inentertainment systems. It is expected that adding haptic communicationto existing virtual-reality and other entertainment systems may enhanceuser experiences.

Users of remote teleoperation devices, such as handling equipment inhazardous environments, may also benefit from haptic communications.

Blind people may also benefit from artificial vision systems thatpresent edge-enhanced images through haptic stimulation devices deployedagainst sensate skin of their back and shoulders.

SUMMARY

A haptic stimulator includes a multilayer sheet with a layer of smartmaterial, for example, either a piezoelectric or electroactive material,adapted to mechanically deform upon application of a voltage to thesmart material, the multilayer sheet secured to a substrate, and asource of electrical stimulation coupled to drive the electrodes on thepolymer layer with an AC signal to vibrate the polymer layer. Inparticular embodiments, the polymer contains polyvinylidene fluoride.

In another embodiment, haptic stimulator has a first and secondelectrode with an air gap and an insulating sheet between first andsecond electrodes, with a high voltage AC driver driving the electrodes.

In a method of providing haptic stimulation to skin an alternatingcurrent signal generator drives a first and second electrode, wherebythe second electrode disposed upon either a piezoelectric orelectroactive polymer sheet, vibrating the polymer layer by driving theelectrodes; and coupling vibrations of the polymer layer to the sensateskin.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross sectional view of a polyvinylidene fluoride (PVDF)piezoelectric structure having electrodes on a top and bottom surface.

FIG. 2 is a cross sectional view of a PVDF piezoelectric structurehaving interdigitated electrodes.

FIG. 3 is a top plan view of a PVDF piezoelectricinterdigitated-electrode structure as fabricated and tested in vivo

FIG. 4 is a cross section of a bimorph PVDF structure with top, center,and bottom electrodes and upper and lower PVDF layer.

FIG. 5 is a cross section of a bimorph-pocket device.

FIG. 6 is a functional block diagram of a system for providing touchsensation to a prosthetic using the polymeric piezoelectric devicesherein described.

FIG. 7 is an outline sketch of a prosthetic embodying the system of FIG.5.

FIG. 8 is a top plan view of an embodiment having center cuts.

FIG. 9 is a perspective view illustrating an embodiment having a spiralKirigami cut permitting additional excursion in vibration magnitude.

FIG. 10 is a top view of a fabricated prototype embodiment having aspiral Kirigami cut similar to that illustrated in FIG. 8.

FIG. 11 is a perspective view illustrating an embodiment havingperipheral cuts permitting additional excursion in vibration magnitude.

FIG. 12 is a perspective view illustrating another embodiment havingperipheral cuts permitting additional excursion in vibration magnitude.

FIG. 13 is a top plan view of a rectangular embodiment supported on onlyone side of the rectangle.

FIG. 14 is a lateral view of the embodiment of FIG. 13 illustratingsignificant displacement of the multilayer sheet because three sides areunsupported.

FIG. 15-17 illustrate several forms of stiff plating for electroactivepolymer (EAP) devices.

FIG. 18 illustrates curling of an EAP device of claim 15.

FIG. 19 is a cross sectional diagram of an electrostatic hapticstimulator.

FIG. 20 illustrates a back pad having a haptic stimulator array.

DETAILED DESCRIPTION OF THE EMBODIMENTS

We propose vibratory haptic-stimulation devices. In embodiments, thesehaptic-stimulation devices are based upon polymeric piezoelectricmaterials. In embodiments mesoporous polyvinylidene fluoride (PVDF) andPVDF copolymers are used as piezoelectric materials in sensors andactuators for haptic communications. These PVDF devices serve both aspressure sensors for use in prosthetics, and as actuators for pressingagainst or vibrating against sensate skin. The sensate skin may be skinof a residual limb or of other parts of the body such as the back ofchest and abdomen.

The biological mechanoreceptors in human touch sensing can detectwhether a material is rough or smooth, hard or soft, sticky or slippery,and at rest or in motion. We aim to simulate human touch with a matrixof simple but scalable actuating elements, where each element isprogrammable for local sensing and actuation. This can assist in virtualreality, to telecommunicate social caring or conversational intentionsthrough touch over a long distance.

The devices have two modes of operation; first, as they respond toexternal forces, such as pressure on a prosthetic glove over aprosthetic hand, they generate voltages that can be sensed. Second, whensufficient high alternating voltages are applied to the PVDF orPVDF-TrFE piezoelectric films, they change shape producing vibrationsthat can be felt by sensate skin operating as haptic skin-stimulationactuators.

In the first mode they can serve as pressure or touch sensors; these canbe embedded in a surface of a prosthetic to serve as touch sensors andthe electronic device can stimulate skin of a residual limb with signalsderived from the pressure or touch sensors. In the second mode they canserve as actuators to stimulate touch sensors in adjacent skin, or,since the piezoelectric film electrical response varies with mechanicalload on these actuators, they can also serve as pressure sensors.

An example of a PVDF copolymer is porous PVDF-TrFE (polyvinylidenefluoride-trifluoroethylene) film is presented in PCT/US15/60342, thecontents of which are incorporated herein by reference.

A bistable structure is one that has a first and a second stablemechanical shape, particularly where they are stabilized by differentcurvature axes. Some embodiments make use of bistable or multistablestructures to provide a pronounced “snap” action when used as sensors.Other embodiments make use of bistable or multistable structures andresulting “snap” action to provide sharper vibratory waveforms withgreater harmonic content when used as haptic skin stimulators.

FIG. 1 is a cross sectional view of a dual-sided-electrodes mesoporouspolyvinylidene fluoride (PVDF) multilayer piezoelectric structure 100,polyvinylidene fluoride being a polymeric piezoelectric material. Asupporting layer 102 is coated and masked with electrode interconnect104, then a bottom electrically-conductive electrode layer 106 isapplied. Bottom electrically-conductive electrode layer 106 andinterconnect 104 are typically metallic. A mesoporous layer ofpolyvinylidene difluoride (PVDF) 108 is then formed atop the conductiveelectrode layer 106. The PVDF film is polarized in a strong electricfield to induce piezoelectricity. Atop the PVDF layer is a topelectrical contact layer 110 and an interconnect layer 112. Aninsulating plastic layer 114 is provided atop the entire structure, inbiological applications the insulating layer is formed of abiocompatible plastic. Supporting layer 102 is chosen to be a moderatelystiff material such as polyimide like Kapton, or polydimethylsiloxane(PMDS), supporting layer. PVDF copolymers have much smaller Young'smodulus than piezoelectric ceramics like lead ziconate titante (PZT) orsolid piezoelectric crystals like quartz, and have relatively highpiezoelectric constants.

In particular embodiments the top and/or bottom electrode contact layersare patterned to provide design engineers with control of electric fielddistributions within the piezoelectric polymer layer.

FIG. 2 is a cross sectional view of a single-sided-electrodes PVDFmultilayer piezoelectric structure 150. A supporting layer 152 is coatedwith a mesoporous layer of polyvinylidene difluoride (PVDF) 158. Atopthe PVDF layer is a top electrode layer forming portions of firstelectrode 160 interdigitated with second electrode 162. In particularembodiments the electrode contact layer pattern is chosen to controlelectric field distributions within the piezoelectric polymer layer. Aninsulating plastic layer 164 is provided atop the entire structure, inbiological applications the insulating layer is formed of abiocompatible plastic.

PVDF Ring-Shaped Interdigitated Transducer

Piezoelectric actuators have been developed using various piezoelectricmaterials and electrode patterns. Ring-shaped interdigitated electrodesare used to excite the film for electrical excitation of the film'sresonance. Frequencies associated with this resonance are determined bymaterial properties and geometric parameters, and vibration amplitudealso depends on the input electric excitation magnitude and electricpatterns. With our polymeric piezoelectric materials, including PVDF,frequencies in the 60 to 200 hertz range are possible.

Previous ring-shaped interdigitated transducers used piezoelectricceramics such as PMN-PT and PZT with resonance frequencies of the basevibrational mode usually higher than 1 kHz, and vibration amplitudes atresonance typically of several micrometers. These vibrations are lessefficient at stimulating skin than the frequencies in the 60 to 200hertz range achieved with polymeric piezoelectric materials.

PVDF and PVDF copolymers, such as PVDF-TrFE (polyvinylidenefluoride-trifluoroethylene) have much smaller Young's modulus thanpiezoelectric ceramics, and have relatively high piezoelectricconstants. Ring-shaped interdigitated transducers (IDTs) based on PVDFfilms excite large out-of-plane vibration amplitude at the resonancefrequency due to the films' relatively low stiffness, and the resonancefrequency can be reduced by orders of magnitude below resonantfrequencies of similar piezoelectric transducers. Thus, PVDF ring-shapedIDTs can form actuators operating at low frequencies and withlarge-displacement mechanical outputs.

A haptic stimulator in form of a piezoelectric device 200 (FIG. 3) withinterdigitated electrodes 202, 204 is formed of the single-sidedelectrodes PVDF multilayer structure 150 mounted to a rectangular frame206. Devices of this type have been fabricated and tested in sizes of 2cm and 2.8 cm diameter, in a rectangular frame sized 3 by 4 centimeters,with resonant frequencies between 80 and 125 hertz, resonant frequencymay in part be controlled by design of thickness of the PVDF film. Asmall air gap is provided beneath the PVDF structure. These devicesproduced vibrations sufficient to stimulate the biological touch sensorsin human skin. The 2 cm diameter device achieved low frequencydisplacement of 80 microns using a PVDF film thickness of 28 microns,under load 5 millinewtons (mN) of force was achieved with 1 kvexcitation voltages. The 2.8 cm diameter device achieved 30 mN of forcewith a 52 micron PVDF film thickness.

At frequencies of 20 and 84 hertz and 500 volt AC stimulation, the 2.8cm device with PVDF film thickness of 52 microns and positioned on anarm of a human volunteer provided sufficient displacement for vibrationsto be felt as of moderate strength, thereby functioning as a hapticstimulator.

Bimorph-Pocket Design

Bimorph is a commonly seen structure employed to generate largedisplacement using a thin plate. The bimorph design uses a multilayerstructure 250 (FIG. 4) having a supporting layer 252, a bottom electrode254, a lower piezoelectric layer 256, and a center electrode 258. Italso has an upper piezoelectric layer 260 and top electrode 262 coveredby a passivation and insulation layer 264. With thin PVDF piezoelectriclayers 256, 260, the bimorph typically generates more force than theembodiment of FIG. 1, but requires a bipolar power supply.

A pocket design is formed by mounting two, back-to-back, bimorphs suchthat the bimorphs 280, 282 (FIG. 5) warp in opposite directions whenactivated by a voltage. In an alternative embodiment, multiple bimorphsare stacked. With five stacked bimorphs we expect to be able to achievemaximum forces of one newton and displacement of 2.5 mm. with 1 kvvoltages, although this level of force and vibration amplitude would bedownright painful if used for haptic communications. With a hapticstimulator having fewer than five stacked bimorphs and reduced voltages,vibration levels felt by a user over a range from barely perceptible tostrong, but not painful, vibration can be achieved.

Prosthetic Sensory System

A system 300 (FIG. 6) has piezoelectric pressure sensors 302, in aparticular embodiment pressure sensors 302 are disposed within aprosthetic glove (not shown for simplicity) adapted to cover a terminaldevice of an upper-limb prosthetic 304 (FIG. 7), the pressure sensorsbeing located over the contact surfaces of the terminal device. Theterminal device may, but need not, have form of an artificial hand—manyprosthetic wearers prefer other forms of terminal devices for particulartasks. The pressure sensors 302 are electrically coupled to a processor310 located in an electronics and battery module 308 of the prostheticby wires (not shown). The processor 310 is configured by firmware in amemory (not shown) to read the pressure sensors 302 and determine anappropriate level of vibration for each of several vibratory hapticstimulators 312, 314 piezoelectric polymer structures, which be thepiezoelectric structures and devices described with reference to FIGS.1-5. In a particular embodiment, haptic stimulators 312 may be locatedin a first array 316 adapted to represent multiple piezoelectric sensors302 located over a contact surface of a first portion 320 of terminaldevice 306 and in a second array 318 adapted to represent multiplepiezoelectric sensors 302 located over a contact surface of a secondportion 322 of terminal device 306. In an embodiment, arrays 316 and 318are located in different portions of a prosthetic liner 326 ofprosthetic 304.

Signals from processor 310 are boosted to the high voltages necessary todrive the haptic stimulators 312, 314 by voltage drivers 330, alsolocated within electronics and battery module 308. In embodiments, liner326 is also equipped with a myoelectric sensor 332 coupled to amyoelectric-controlled motor driver (not shown) in electronics andbattery module 308, the motor driver coupled to drive a motor 334configured to operate terminal device 306.

In alternative embodiments, the haptic stimulators 312, 314, 600 may belocated in a back pad 650 (FIG. 20), such a back pad may be particularlyappropriate for use with a synthetic vision system for the blind.

Kirigami PVDF Devices

By using techniques from kirigami (the art of folding and cuttingpaper), we can selectively remove unnecessary parts of the film and makecuts to focus displacement in certain regions and to permit greaterdisplacements than available with an intact, uncut, film.

Test devices were formed of PVDF film with metallic electrode patternsof gold sputtered onto top and bottom surfaces of the film using masks.This pattern determines which parts of the device contribute bothelectrically and mechanically to displacement, and which partscontribute only mechanically. A late step of manufacture is the kirigamicut, in which certain pieces of the film are excised to improve theoverall device motion. We considered three kirigami-cut embodiments,spiral-cut, center-cut, and peripheral-cut.

Spiral Cuts

An embodiment 348 (FIG. 9) has a spiral kirigami cut 350 permittingadditional excursion in vibration magnitude beyond that obtained withuncut haptic stimulators. A spiral-cut piezoelectric structure has beenfabricated and tested; see the photo in FIG. 10. The number of turns andwidth of the spiral cut significantly affect the resonant frequencies ofthe device, permitting adjustment of a design for a particular resonantfrequency.

Center (Diagonal) Cuts

In a center-cut or diagonal-cut embodiment 400 (FIG. 8) the multilayerpiezoelectric sheet with top and bottom electrodes is formed as arectangle having all four perimeter edges attached to the substrate, thepolymer layer has a first slit 402 positioned along a line defined by anintersection 404 of the first and second edges to an intersection 406 ofthe third and fourth edges, and a second slit 408 positioned along aline defined by an intersection 410 of the second and third edges to anintersection 412 of the first and fourth edges, the first and secondslit intersecting near a center of the rectangle.

The center-cut embodiment permits the four points, such as point 414, toflex more freely than an uncut embodiment because the points canseparate freely from each other as they lift toward their highestposition. The result is increased vibration magnitude over an uncutembodiment while retaining more of the vibration force than typical witha spiral-cut embodiment.

Peripheral Cuts

In another embodiment 450, 460 (FIGS. 11 and 12), in order toeffectively reduce stiffness of the multilayer sheet and increasevibrational magnitude, without serious reduction in vibratory force, thedevice center is left without cuts, but multiple holes or cuts 452, 462are positioned around a periphery of the sheet between device center470, 472 and device edges.

Frequency Range

Embodiments of haptic stimulators as herein described have resonances,and may therefore be efficiently operated at, low frequencies below 200hertz. In particular embodiments, the devices are efficient whenoperated in the frequency ranges from 5 to 200, from 60 to 200, from 20to 84, or from 80-125 hertz.

Cantilevered Embodiment

In an embodiment 500 (FIG. 13) the multilayer sheet is, unlike allpreviously described embodiments, supported only on one side 504 of therectangular space allocated to each piezoelectric haptic stimulator.With the remaining three sides floating, the entire rectangle can flex,as seen in FIG. 14. It is believed that this embodiment could provide600 mN of force and a millimeter of displacement with a one-centimetersquare haptic actuator.

Electroactive Perimeter Frame

We experimented with using kirigami patterning (FIG. 15-17) to createstiff plating for electroactive polymer (EAP) devices. EAPs contractvertically and expand horizontally under application of high voltage. Byfixing prestretched EAP films onto these kirigami backings, the devicescould be induced to curl into specific shapes, as shown in FIG. 18, dueto strain mismatch. When the EAP films were actuated, they would unfurlto a flatter state, dictated by the shape and size of the backing. Themore material is removed from the backing, the higher the displacementand lower the force of the resulting device.

The soft material of electroactive polymers (EAPs), which deforms inresponse to application of an electrical stimulus, the forces induced bythe EAPs may be used to trigger the snap-through of bistable structures.Preliminary results are applicable to both multi-stable structures. FIG.15-18 give a few designs on EAP based actuators.

Electrostatic Haptic Stimulator

An electrostatic haptic stimulator 600 (FIG. 19) has a first electrode602, a second electrode 604 formed of an electrically conductive layerdisposed on a pre-deformed, domed, insulating sheet 606, with the secondelectrode configured parallel to and with an air gap from the firstelectrode. There is an insulating sheet 608 between the second electrodeand the first electrode to prevent their shorting together. Thestimulator 600 is driven by a high voltage source coupled through leadwires 610, 612, the lead wires contacting the electrodes. The topinsulating sheet 604 is drawn downward by electrostatic forces when highvoltage is present between the electrodes, and will therefore vibratewhen driven by an AC voltage. The top insulating sheet 604 may directlycontact sensate skin or may transfer vibration through a soft,overlying, insulator such as a glove, sock, or liner.

A prototype of the electrostatic haptic stimulator of FIG. 19 has beenobserved to provide ten mN of force when driven with 800 volts at 10hertz, with approximately 0.4 mm of vibrational magnitude—sufficient tobe felt and to serve as a haptic stimulator.

By charging different parts of the upper or second electrode, this airgap can be localized to a specific portion of the device, and can befurther moved controllably around the device by precisely switching thecharging areas. Finally, when a load is placed on top of the device, themotion of the gap will transform into shear force. This technology canbe seamlessly integrated into a wearable device to generate a feeling ofshear motion on human skin.

In a particular embodiment, the top insulating sheet is configured to bebistable, so it retracts and pops outwards with distinct sharp motions.

In yet another embodiment, an electrostatic device of FIG. 19 iscombined with a PVDF-TrFE layer device like that of FIG. 4, can bedesigned for both sensing and actuation, owing to the fact thatPVDF-TrFE films can act as excellent pressure sensors. Specifically, athin layer of piezoelectric PVDF can be placed underneath an airelectrostatic gap device. When the device is compressed, the PVDF-TrFEwill generate a voltage which can be extracted and correspond to acertain sensing level. This allows the device to act both as an actuatorand a sensor with minimal increase in thickness or complexity.

In this invention, the air gap device using electrostatic mechanism canact as a force generator, and the sensing function is realized byutilizing the piezoelectric material. The array of air gap devices canincrease the sensation level on human skin with simple designedstructure by driving both the piezoelectric and electrostatic actuators.

Changes may be made in the above methods and systems without departingfrom the scope hereof. It should thus be noted that the matter containedin the above description or shown in the accompanying drawings should beinterpreted as illustrative and not in a limiting sense. The followingclaims are intended to cover all generic and specific features describedherein, as well as all statements of the scope of the present method andsystem, which, as a matter of language, might be said to falltherebetween.

What is claimed is:
 1. An apparatus configured to stimulate sensate skin, the apparatus comprising: a multilayer sheet comprising a polymer layer adapted to mechanically deform upon application of a voltage to the polymer, the multilayer sheet secured to a substrate, the polymer being piezoelectric polyvinylidene fluoride; a first and a second electrode disposed upon the polymer layer, and a source of electrical stimulation coupled to drive the first and second electrode with an alternating current signal to vibrate the polymer layer; wherein the first and second electrode are disposed on a same side of the polymer layer, the first and second electrodes being interdigitated.
 2. The apparatus of claim 1 wherein the first and second electrode are configured in a circular pattern of between two and three centimeters diameter.
 3. The apparatus of claim 1 wherein the alternating current signal has a frequency corresponding to a vibrational resonance of the film.
 4. The apparatus of claim 3 wherein the frequency is between 60 and 200 hertz.
 5. A method of providing haptic stimulation to sensate skin comprising: coupling an alternating current signal generator to drive a first and second electrode, the first and second electrode disposed upon a polymer layer comprising polyvinylidene fluoride, the polymer layer, first electrode and second electrode comprising a multilayer sheet; and vibrating the polymer layer by enabling the alternating current signal generator; and coupling vibrations of the polymer layer to the sensate skin; wherein the first and second electrode are disposed on a same side of the polymer layer, the first and second electrodes being interdigitated.
 6. The method of claim 5 wherein the first and second electrode are configured in a circular pattern of between two and three centimeters diameter.
 7. The apparatus of claim 5 wherein the alternating current signal has a frequency corresponding to a vibrational resonance of the film.
 8. The apparatus of claim 7 wherein the frequency is between 60 and 200 hertz. 